We have recently been studying the synthesis of glycoproteins and their lipid intermediate precursors in cell free systems. We have found that the pattern of mannosyl lipid-linked oligosaccharides synthesized by cell-free enzyme preparations from cultured fibroblasts is altered substantially when 0.2 micromolar UDP-glucose is added to the incubation medium. Inclusion of UDPG results in the appearance of a new labelled oligosaccharide, which is one or two glycose units larger than the lipid-linked oligosaccharide synthesized in the presence of only GDP-mannose (2 micromolar) and UDP-N-acetylglucosamine (20 micronmolar). Label from UDP-3H glucose is incorporated into the same larger oligosaccharide size class. It seems likely that the new mannosyl-lipid-linked oligosaccharide contains one or two glucose residues in addition to 5-6 mannose residues. The results are compatible with the recent finding by Spiro et al. that naturally occurring mannosyl lipid linked oligosaccharides contain glucose. In addition to being incorporated into lipid oligosaccharides, glucose residues are also incorporated into endogenous glycoproteins. Incorporation of glucose into glycoproteins that give rise to Pronase glycopeptides of the typical asparagine linked size classes is almost completely dependent on the presence of GDP-mannose. Taken together, the results and systems discussed above suggest some obvious directions for research in the coming year. Those that we are emphasizing are: 1) Structural studies on the glucose-mannose containing lipid oligosaccharides and glycopeptides. 2) Studies on the transfer of isolated lipid linked oligosaccharides to endogenous and exogenous protein acceptors. 3) A search for "maturation" reactions which alter the carbohydrate structure of glycoporteins formed in vitro. BIBLIOGRAPHIC REFERENCES: A New Procedure for the Preparation of GDP-U 14C Mannose. W.A. Braell, M.A. Tyo, S.S. Krag, and P.W. Robbins, Analytical Biochemistry 74, 484 (1976).